This invention relates to friction clutches of the type employed in drive lines of heavy duty vehicles. More particularly, the invention relates to friction facing elements employed in the driven discs of such friction clutches.
Traditional friction elements of the type bonded and/or riveted to clutch discs have been made primarily of organic fiber materials. Such materials have been satisfactory under normal conditions wherein the friction elements have had standard useful lives. However, in heavy duty environments, the organic friction elements have tended to wear out in considerably shorter periods of time and to thus require frequent replacement. This problem has become exacerbated in modern heavy duty vehicles wherein even greater loads have been imposed on such elements.
The latter has led to the development of friction elements which employ ceramic facings to provide greater durability. However, in the environment of friction clutches, ceramic materials have been found to be generally sensitive to heat, in that their coefficients of friction vary as a function of temperature. Thus, while the coefficient of friction of a ceramic element may have a value of 0.3 over a surface area thereon at a temperature of 350.degree. F., the same element may on a separate localized area have a coefficient of friction of 0.5 at 600.degree. F. Such disparity in coefficient of friction values over the surface of a friction element causes a chatter phenomenon, wherein the element may rapidly alternately grab and release an associated pressure plate during clutch engagement.
What is needed is a ceramic friction clutch element which is less sensitive to heat, and thus not subject to the aforedescribed chatter phenomenon.